1. Field of the Invention
The present invention relates to an organic electro-luminescence display, and more particularly to an apparatus and a method for driving an organic light-emitting diode.
2. Description of the Related Art
Recently, various flat panel display devices have been developed, which are light, thin, and capable of resolving shortcomings of cathode ray tubes (CRT). Examples of these panel display devices include liquid crystal display (LCD), field emission display (FED), plasma display panel (PDP) and electro-luminescence (EL) display.
The EL display is a self-luminous device capable of light-emission by a re-combination of electrons with holes in a phosphorous material. EL displays are generally classified into inorganic EL display devices and organic EL display devices, depending on material and structure. An EL display provides similar advantages to the CRT. For example, the EL display has a faster response time than a passive-type light-emitting device, such as an LCD, which requires an additional light source.
FIG. 1 is a cross-sectional view of an organic EL structure for describing the operation of a light-emitting diode according to a related art. Referring to FIG. 1, the organic EL device of the EL display (ELD) includes an electron injection layer 4, an electron carrier layer 6, a light-emitting layer 8, a hole carrier layer 10, and a hole injection layer 12 that are sequentially disposed between a cathode 2 and an anode 14. The anode 14 can be a transparent electrode. The cathode 2 can be a metal electrode.
If a voltage is applied between the anode 14 and the cathode 2, electrons generated at the cathode 2 flow into the light-emitting layer 8, via the electron injection layer 4 and the electron carrier layer 6, while holes generated at the anode 14 flow into the light-emitting layer 8, via the hole injection layer 12 and the hole carrier layer 10. Thus, the electrons and the holes fed from the electron carrier layer 6 and the hole carrier layer 10, respectively, collide and recombine within the light-emitting layer 8 and generate light. Then, the light generated by the recombination of electrons in the light-emitting layer 8 is emitted out of the light-emitting diode, via the transparent electrode (i.e., the anode 14). Thus, a picture can be displayed using a plurality of such light-emitting diodes.
FIG. 2 is a schematic block diagram of an organic electro-luminescence display device according to the related art. Referring to FIG. 2, the related art organic EL display device includes an EL display panel 16 having a plurality of pixel cells PE forming a matrix. The pixel cells are located at pixel areas defined by crossings of scan electrode lines SL1 to SLn and data electrode lines DL1 to DLm. A scan driver 18 is provided for driving the scan electrode lines SL1 to SLn. A data driver 20 is provided for driving the data electrode lines DL1 to DLm. A timing controller 28 controls the timing for driving the gate driver 18 and the data driver 20.
FIG. 3 shows a cell driving circuit for driving a pixel cell in the organic electro-luminescence device according to the related art. Referring to FIG. 3, each pixel cell PE includes an organic light-emitting diode OLED and a light-emitting diode driving circuit 30. The organic light-emitting diode OLED is connected between a supply voltage line VDD and a ground GND. The light-emitting diode driving circuit 30 drives the light-emitting diode OLED in response to a driving signal supplied from each of the data electrode lines DL and the gate electrode lines SL.
More specifically, the light-emitting diode driving circuit 30 includes a driving thin film transistor (TFT) DT connected between the supply voltage line VDD and the light-emitting diode OELD, a switching TFT SW connected to the scan electrode lines SL, the data electrode lines DL and the driving TFT DT, and a storage capacitor Cst connected between a first node N1 positioned between the driving TFT DT and the switching TFT SW, and the supply voltage line VDD. Herein, the TFT is a p-type electron metal-oxide semiconductor field effect transistor (MOSFET).
A gate terminal of the driving TFT DT is connected to a drain terminal of the switching TFT SW. A source terminal of the driving TFT DT is connected to the supply voltage line VDD. A drain terminal of the driving TFT DT is connected to the light-emitting diode OLED.
A gate terminal of the switching TFT SW is connected to the scan electrode line SL. A source terminal of the switching TFT SW is connected to the data electrode line DL. A drain terminal of the switching TFT SW is connected to the gate terminal of the driving TFT DT.
The timing controller 28 generates a data control signal for controlling the data driver 20 and a scan control signal for controlling the scan driver 18. The timing controller 28 uses synchronizing signals supplied by an external system, for example a graphic card. Further, the timing controller 28 applies a data signal from the external system to the data driver 20.
The scan driver 18 generates a scanning pulse SP in response to the scanning control signal from the timing controller 28. The scan driver 18 applies the scanning pulse SP to the scan electrode lines SL1 to SLn to sequentially drive the scan electrode lines SL1 to SLn.
The data driver 20 supplies a data voltage to the data electrode lines DL1 to DLm every horizontal period H in response to the data control signal from the timing controller 28. The data driver 20 has output channels 21 that are in one-to-one correspondence with the data electrode lines DL1 to DLm.
In each pixel cell PE of the related art EL display device, if a scanning pulse SP having a LOW state is inputted from the scan driver 18 to the scan electrode line SL, then the switching TFT SW is turned on. When the switching TFT SW is turned on, a data voltage supplied from the data driver 20 to the data electrode line DL is applied, via the switching TFT SW, to the first node N1 in synchronization with the scanning pulse SP applied to the scan electrode line SL. The data voltage applied to the first node N1 is stored in the storage capacitor Cst.
The storage capacitor Cst stores the data voltage from the data electrode line DL during the time the scanning pulse SP is applied through the scan electrode line SL. The storage capacitor Cst holds the stored data voltage during one frame period. In other words, the storage capacitor Cst applies the stored data voltage to the driving TFT DT when the scanning pulse SP is not applied to the scan electrode line SL, to thereby turn on the driving TFT DT. Thus, the light-emitting diode OLED is turned on by a voltage difference between the supply voltage line VDD and the ground GND. The light-emitting diode emit light in proportion to the intensity of current flowing from the supply voltage line VDD through the driving TFT DT.
In the related art EL display device having such a structure, a device characteristic between the interior of the panel and the panel is non-uniformly formed due to instability in a laser output power during a polysilicon crystallization process. The output current of the driving TFT DT in response to the same data voltage changes because of the non-uniformity in the characteristics of the device. The pixel structure of the conventional EL display device fails to compensate for a non-uniformity in picture quality caused by the non-uniform characteristic of the driving TFT DT between the panel and its interior.